1. Field of the Invention
The present invention relates to the indirect heating of an element by a heat source using a circulated heat exchange medium guided to avoid stagnation of flow in the body of the medium. More specifically, this invention relates to arranging the heat source, the element to be heated, and baffles to avoid conflict between the rising streams of the heated portions of the medium and the sinking streams of the cooled portions of the medium as the medium circulates between the heat source and the element and to thereby maximize the circulation and consequently the heat transfer between the heat source and element.
2. Description of the Prior Art
Convective heat transfer is a standard design consideration in equipment in which heat is transmitted by a fluid medium from a heat source to a structure to be heated. In broad principle there is no present mystery in the transmission of heat by exposing as much surface area as feasible at both the heating of the medium by the source and the heating of the structure by the medium. Next, it is accepted that uniform distribution of the medium over the heat exchange surfaces is a key to the effectiveness and efficiency of this heat exchange. Also, it is recognized that the medium must be moved dynamically over both heat exchange surfaces to militate against stagnation.
The prior art is replete with examples of how these basic factors are given weight in the design of heating units. To pick an example, the disclosure of U.S. Pat. No. THURLEY 2,993,479 issued July 25, 1961, shows the discharge of products of combustion from a primary furnace and these products being guided over heated tubes. The products of combustion became the fluid medium which is directed by means of baffles into uniform, dynamic contact with the tubes to be heated.
The U.S. Pat. No. WALKER et al 2,354,932 issued Aug. 1, 1944, disclosed a liquid heat transfer medium for conveying heat from a heat-supplying element to a heat-absorbing element, both of which are submerged in the medium. A tank 1 is divided by a horizontal partitioning member 8 which separated the tank into upper and lower compartments 9 and 10. Firetube 19 is in the lower compartment 10. Coil 28 is in the upper compartment 9. Liquid heat transfer medium 40 fills the remainder of the tank.
The heat liquid beneath partition 8 rises and is guided by a baffle up one end of partition 8. The elevated heated liquid then flows over coil 28 which extracts heat from the medium. The cooled liquid then sinks and is guided by a baffle down the second end of partition 8 to complete the thermo-syphonic cycle. It was expected that this flow would have a pattern that dynamically eliminates stagnation and maximizes heat exchange.
However, it is apparent that the medium flows parallel the lengths of both the firetube 19 and coil 28. It is fundamental that the rate of heat exchange is less in this parallel flow arrangement than in transverse flow. In addition, the necessary support and/or spacing structure for the tubes inherently impedes the flow of the medium over the tube surfaces. As the thermal driving force on the liquid medium is upward, the partition 8 over most of its length directs the flow of the medium perpendicular to this driving force. The hope for more efficient operation due to the disclosed arrangements gradually faded.
It was years before the limitations in the theories of the WALKER et al U.S. Pat. No. 3,354, 932 were completely evaluated. All the literature of the Assignor, National Tank Company, featured the disclosure up into the 1960's. The partition 8 was finally eliminated from the literature and fabricated units. Today, the coil to be heated is simply mounted above the firetube. But this arrangement does not solve the problem.
The present arrangement of coil above firetube is more easily analyzed. And problems are evident. The thermal flow force on the medium by the firetube is upward. The thermal flow force on the medium by the coil is downward. Obviously, these flow patterns are in opposition and with no means of controlling or separating the flow patterns there is stagnation. A new form of baffle is required to resolve this conflict of flows.
More specifically, the upward flow path for the heated liquid must be preserved and the downward flow path for the cooled liquid must be provided, and baffles are needed to guide these liquid flows into separate channels or patterns. The thermal flow patterns or forces must not directly conflict or the resulting opposition and stagnation will prevent maximum heat exchange.